1. Field of Invention
This present invention relates to the general area of insonification devices that are designed to generate a beam of focussed waves, especially in the ultrasound frequency domain.
2. Background of the Invention
Such insonification devices, very frequently called probes, are used to perform the imaging of an environment or a medium, or indeed to modify the properties of this environment. For example, an insonification device according to the invention can be used to raise the temperature of a very precise zone corresponding to the focus zone, where one wishes to focus the beam of ultrasound waves. These devices can be used in particular not only in the medical field but also in the areas of underwater imaging and communication, imaging of the terrestrial subsoil, or indeed in the area of non-destructive testing of materials.
In order to achieve focussing, one is familiar with the use of N ultrasound sources that are controlled independently in phase and in amplitude by electronic means, so that their overall energy reaches a local maximum in a predetermined focus zone.
The positioning of the transducers, in relation to the environment, and in relation to the focus zone, is important since it ensures the effectiveness and the safety of the action performed by virtue of the insonification device, to image or to modify the properties of the environment. In fact the field diffracted by all of the transducers can be modified significantly in accordance with the positioning of the transducers and their geometry.
Thus, when the layout of the transducers exhibits symmetries, this results in the presence of significant side lobes. This reduces the quality of the focus. In particular, points that are outside of the focus zone can then receive significant quantities of energy, when in fact these zones should not be subjected at all to the energy emitted by the insonification device.
Thus, the document entitled “Sparse Random Ultrasound Phased Array for Focal Surgery”, Goss et al., IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 43, No. 6, November 1996, proposes to generate a network of transducers in a virtually random manner over a three-dimensional concave area, the only condition being to maintain[S1] of a minimum distance between the different transducers.
The three-dimensional concave area is used to perform pre-focussing prior to the additional electronic focussing that will then be performed by applying individually calculated phase delays to each transducer. This application of phase delays is commonly employed in the ultrasound area. These controls are achieved by means of a limited number of independent electronic channels, which also limits the number of transducers that can be deployed in the insonification devices used by the invention.
The random distribution of the transducers, as described in the aforementioned document, nevertheless has the drawback of proposing an inhomogeneous layout for all of the transducers in the three-dimensional concave area. In fact certain transducers can be placed very close and others very far apart. This leads to a locally heterogeneous density of the energy emitted over all of the emission area.
Such heterogeneity of the pressure field generally leads to the appearance of acoustic energy maxima, also called “hot spots”. These maxima can be large at the entry of the ultrasound beam into the environment, and can give rise to irreversible and undesirable damage to the environment or to its surface. They can also be the source of parasitic waves returned by the environment during imaging of the latter.
The side lobes appear whenever the transducers, which are not infinitely small, are separated by a distance greater than some fractions of wavelengths. The limited number of transducer control channels, and the need to create a probe of sufficiently large size, necessarily results in the appearance of unwanted hot spots.
Such a problem is exacerbated by the shape of the three-dimensional concave area, which has been precisely designed to achieve a focus.